Method of producing doped tungsten powders by chemical deposition

ABSTRACT

Method of preparing doped tungsten powders for use in the fabrication of tungsten filament wire by simultaneous chemical vapor deposition of tungsten and dopant. There is also provided the product produced by this process which has a very homogeneous distribution of dopant within individual particles of powder.

United States Patent 11 1 Brecher et a1. Nov. 20, 1973 [54-] METHOD OF PRODUCING DOPED 2,966,406 12/1960 Spier et al. 75 .5 AB TUNGSTEN POWDERS BY CHEMICAL 3,062,638 11/1962 Culbertson et al. 75/.5 BB 2,949,358 8/1960 Alexander et 75/.5 AC X DEPOSITION 2,828,199 3/1958 Findlay 75/.5 BB X [75] Inventors: Lee E. Brecher; Roland Stickler, 3,671,220 6/1972 .lonsson 75/.5 B both of Pittsb r h, P 3,450,525 6/1969 Van Den Steen 75/.5 B X [73] Assignee:- Westinghouse Electric Corporation,

Pittsburgh, Pa. Primary Examiner-G. T. Ozaki Filed Dec 22 1971 Attorney-A. T. Stratton et a1.

[21] Appl. No.: 210,952

[57] ABSTRACT 75/.5 BB, 75/.5 Method of preparing doped tungsten powders for use 58 d 5 AB in the fabrication of tungsten filament wire by simulta- 1 le 0 5 5 neous chemical vapor deposition of tungsten and dopant. There is also provided the product produced by this process which has a very homogeneous distribu- [56] References Clted tion of dopant within individual particles of powder.

UNITED STATES PATENTS 3,341,320 9/1967 Smiley 75/.5 BB 5 Claims, 2 Drawing Figures FIG.I

FIG. 2

HALIDE GAS SOURCE OF GAS COMPRISING AT LEAST THE METALLIC CONSTITUENTS OF THE DOPANT INTRODUCING TUNGSTEN INTO THE DEPOSITION CHAMBER TO REACT IT WITH THE HYDROGEN TO CONSTITUENTS OF THE DOPANT TO PARTICLES OF TUNGSTEN POWDER SOURCE OF GAS HEATING THE TUNGSTEN-HALIDE GAS CONTINUING TO INTRODUCE THE GASES UNTIL A PREDETERMINED QUANTITY OF DOPED TUNGSTEN POWDER HAS BEEN OBTAINED TUNGS TEN- HALIDE PRECIPITATE PARTICLES OF TUNGSTEN POWDER H2 GAS SIMULTANEOUSLY INTRODUCING GAS CONTAINING INTRODUCING HYDROGEN INTO A DEPOSITION CHAMBER CAUSE DOPANT TO BE INCLUDED IN THE FORMED SOURCE OF PAIENIEI] IIUV 20 I975 llllllll /r METHOD OF PRODUCING DOPED TUNGSTEN POWDERS BY CHEMICAL DEPOSITION CROSS REFERENCE TO RELATED APPLICATION In copending US. application, Ser. No. 210,951 filed Dec. 22, 1971 by D. H. Archer, L. E. Brecher, J. P. Morris, F. Talko and H. L. Taylor, and owned by the present assignee, isdisclosed a method of preparing tungsten, molybdenum, or zirconium members by a chemical vapor deposition process wherein the unitary member is produced by a continuous process and/or a process where tungsten and dopant are simultaneously or alternately deposited on the member.

BACKGROUND OF THE INVENTION This invention relates to the preparation of doped tungsten for use in the fabrication of lamp filaments and, more particularly, the preparation of doped tungsten powder by chemical vapor deposition.

Chemical vapor deposition as used herein, is a chemical reaction between gases, as opposed to the thermal decomposition of a tungsten halide which occurs in the wellknown tungsten-halogen cycle to deposit tungsten on a lamp filament. The decomposition in the tungstenhalogen cycle is described in US. Pat. No. 2,883,571 issued to Fridrich et al. on Apr. 21, 1959.

Doped tungsten is normally fabricated by a power metallurgy process. In this process the tungsten ore is converted to a purified oxide in powdered form. The powdered tungsten oxide is doped with small amounts of K, Al, and Si compounds and then reduced to tungsten metal powder. The mixture of powders is then compacted and sintered into a high density mass or sintered ingot. The ingot is then swagged down, typically by many steps each of which successively reduce the cross-sectional area. The material is then further reduced by drawing, again typically through a series of successive reductions. The working of the doped tungsten, by swagging and later by drawing, distributes the dopant particles somewhat more evenly and thereby reduces offsetting and sagging of the wire when used as a filament. Even if conventional wire is heavily worked, however, the dopants are not distributed in the tungsten in a homogeneous manner. Even after heavy working, the remaining nonhomogeneity of the wire renders it more likely to fail either by offsetting or by sagging, or by formation of hot spots, leading to failure by localized melting.

The process of this invention produces tungsten metal powder directly and eliminates the conventional step of reduction of the tungsten oxide. This process also provides a very homogeneous doped tungsten which is less likely to fail by offsetting, sagging, or by hot spot'formation and which requires less working of the wire to provide lamp filament quality wire.

In US. Pat. No. 3,510,291 dated May 5, 1970, one process is disclosed by which undoped tungsten metal powder is formed from an aqueous solution of ammoniated tungsten compound by atomizing the solution into a chamber with a hydrogen atmosphere maintained at least 750 C, and a second process is disclosed by which tungsten oxide powders may be formed. To provide filament quality wire, by either of the processes of the aforementioned patent, dopant powders have to be mixed with the resulting tungsten powder.

SUMMARY OF THE INVENTION This invention provides an improved method of preparing doped tungsten powder for use in lamp filament wire wherein: hydrogen is introduced into a deposition chamber; tungsten-halide gas is introduced into the deposition chamber; the tungsten-halide gas is heated to at least 450 C to react the tungsten-halide with the hydrogen and cause the tungsten constituent of the tungsten-halide gas to precipitate out as particles of tungsten powder; simultaneously a predetermined concentration of gas comprising at least the metallic constituents of a preselected dopant is introduced into the deposition chamber thereby causing the dopant to be included in the particles of tungsten powder in a ratio of dopant to tungsten of l-500 ppm; and the hydrogen, the tungsten-halide gas, and the gas comprising the metallic constituents of the dopant are continually introduced until a predetermined quantity of precipitated tungsten powder has been obtained.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference may be had to the preferred embodiment, exemplary of the invention, shown in the accompanying .drawing in which: 1

FIG. 1 is a flow chart illustrating the method steps used to prepare doped tungsten powder for use in the fabrication of lamp filaments.

FIG. 2 is a schematic diagram illustrating the configuration of the preferred system wherein an electric spark is used to initiate the halide-hydrogen reaction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1 there is shown a flow chart of the method steps used to produce filament quality tungsten powder in accordance with this invention. Hydrogen is introduced into a deposition chamber. Tungsten-halide gas is also introduced into the deposition chamber. The quantities of H and halide are selected so that the mol ratio of H to halide is at least 3. The tungsten-halide gas is then heated to at least 450 C to react the halide with the hydrogen and cause the precipitation of particles of tungsten powder. Tungsten may be deposited from a halide or an oxyhalide. The use of the oxyhalide provides water vapor as one of the reaction products and is one method of providing the water vapor which is used in some of the doping reactions. The following equations are examples of some of the reactions which may be used to precipitate particles of tungsten powder:

The gas comprising the metallic constituents of the dopants is simultaneously introduced such that dopant is included inthe particles of tungsten powder. The following equations describe reactions which individually or in combination may be used to deposit dopant:

ZrCl 21-1 ZrO 4HCl ThCl 21-1 0 Th0: 4l-ICl ZAIC]; 31-1 0 Al O 6I-IC1 SiCl, 21-1 0 SiO 4HCl The following reactions may also directly or indirectly occur:

K(vapor) K(dissolved in tungsten) 1((vapor) I'ICl KCl (in tungsten) AH,

2KC1+ 11,0 K 0 21-1Cl The exact reaction when potassium is introduced either in the form of KCl, or K(vapor) is not known but analysis of the resulting tungsten has shown that potassiumdoped tungsten has been produced when either KCl or 1((vapor) was introduced. Sodium or lithium can be introduced in a manner similar to potassium. For doped tungsten deposition the pressure in the chamber is maintained at about 5-800 torrs. In order to prevent condensation of any of the gases used, the chamber is maintained at at least 300 C when AlCl or SiCl is used, and is maintained at at least 100 C when any other dopants described herein are .used without AlCl or SiCl,. Hydrogen gas, tungsten-halide gas, and the gas comprising the metallic constituents of the dopants can be continually introduced to continue the production of precipitated doped tungsten powder.

With reference to FIG. '2, there is shown a deposition chamber in which precipitated doped tungsten power 12 is produced by the preferred method of this invention. The .flow of gas from'the H source 14 iscontrolled to a preselectedflow rate by a first valve 16, the

flow of gas from the source of tungsten-halide gas 18 is controlled to a preselected flow rate by a second valve 20, and the flow of gasfrom the source of gas comprising at least the metallic constituents of the dopant 22 is controlled to a preselectedflow rate by a third valve 24. These preselected flow rates provide predetermined concentrations of the gases and maintain a mo] ratio of H, to halide of preferably 10-20 in the ,decomposition chamber 10.

The halide-hydrogen reaction is'initiated by a spark generated by an electric spark means 26.'Be'cause the reaction of the tungsten halide and hydrogen is highly exothermic, it is not necessary to continue to supply heat from an external source once the reaction has begun.

The halide-hydrogenreaction maybe initiated as an alternative to the electric spark means 26, by other means such as by'a heated filamentor by preheating one of the gases to over450 C.

The following specific examples will describe in greater detail various combinations which are possible using this method:

EXAMPLE] A deposition at a rate of about 13.3 grams per hour of tungsten powder doped with silica, alumina, and potassium is'obtained as follows: a chamber temperature of 300 C, a flow rate of SiCl, of 0.0014 standard cubic centimeters (scc) per minute, a flow rate of AlCl of 0.073 gram per minute, a flow rate of. potassium of 0. l 2 gramper'minute, a flow rate of hydrogen of 1,430 scc per minute, a flow rate of water vapor of 2 sec per minute, 'a chamber pressureof approximately 5 torrs, and a flow rate of WCl of 052 gram per minute.

EXAMPLE II A deposition rate of about 66.5 grams per hour of tungsten powder doped with thoria is obtained as follows: a chamber temperature of approximately C, a chamber pressure of 20 torrs, a flow rate of hydrogen of 7,150 scc per minute, a flow rate of water vapor of 10 sec per minute, a flow rate of WF of 2 grams per minute, and a flow rate of ThCl, as produced by sec per minute of helium flowing through a canister containing ThCl at F and at one atmosphere.

EXAMPLE III Zirconium doped tungsten is produced by the procedures specified in Example 11 by replacing the 'IhCl with ZrCl.,.

EXAMPLE V A deposition rate of about 133 grams per hour of tungsten powder doped with potassium is obtained as follows: a chamber temperature of 250 C, a chamber pressure of 800 torrs, a flow rate of hydrogen of 14,300 scc per minute, a flow rate of potassium of 1.2 grams per minute, and a flow rate of WCl of 5.3 grams per minute.

All of the depositions in the preceding examples are calculated to give tungsten doped with about the following concentration of dopant, when that dopant is included in the composition: silicon 1 ppm, and all other dopants in amount of 100 ppm each. The flow rate of the gas comprising the metallic constituents of the dopant can be adjusted provided a predetermined concentration of the gas in the deposition chamber and thereby vary the concentration of any of the dopants in the precipitated tungsten powder in the range of 1-500 1 1 We claim:

1. A method of preparing a tungsten powder containing a predetermined concentration of preselected dopant from which lamp filament wire can be prepared, which method comprises:

a. introducing hydrogen into a chamber;

b. introducing a predetermined concentration of tungsten-halide gas into said chamber;

0. heating said tungsten-halide gas to at least 450 C in the presence of said hydrogen to react said tungsten-halide gas with said hydrogen and cause the tungsten constituent of said tungsten-halide gas to precipitate as particles of tungsten powder;

d. simultaneously introducing into said chamber a predetermined concentration of gas comprising at least the metallic constituents of said preselected dopant to cause said dopant to be included in said precipitated particles of tungsten powder in a ratio of dopant to tungsten of 1-500 ppm; and

e. continuing the introduction of said hydrogen, said tungsten-halide gas, and said gas comprising at least the metallic constituents of said preselected dopant until a predetermined quantity of doped tungsten powder has been obtained.

2. The method as specified in claim 1, wherein:

a. said tungsten-halide gas consists essentially of at least one of WF and WCl b. said gas comprising at least the metallic constituents of said preselected dopants consists essentially of water vapor and at least one of AlCl SiCl,, ZrCl ThCl and K;

4. The method as specified in claim 3, wherein said gas comprising at least the metallic constituents of said preselected dopant consists essentially of AlCl SiCl and K 5. The method as specified in claim 3, wherein the gas comprising at least the metallic constitutents of said preselected dopant consists essentially of K. 

2. The method as specified in claim 1, wherein: a. said tungsten-halide gas consists essentially of at least one of WF6 and WCl6; b. said gas comprising at least the metallic constituents of said preselected dopants consists essentially of water vapor and at least one of AlCl3, SiCl4, ZrCl4, ThCl4, and K; c. said chamber is at a pressure of 5-800 torrs; and d. flow rates of H2 and halide are preselected to maintain a mol ratio of H2 to halide of at least 3 in said chamber.
 3. The method as specified in claim 2, wherein said heating of the halide gas is initially provided by an electric spark and is thereafter provided by the exothermic reaction between said halide and said hydrogen.
 4. The method as specified in claim 3, wherein said gas comprising at least the metallic constituents of said preselected dopant consists essentially of AlCl3, SiCl4, and K.
 5. The method as specified in claim 3, wherein the gas comprising at least the metallic constitutents of said preselected dopant consists essentially of K. 